This invention relates to electromechanical optical modulators and, in particular, to an optical modulator providing enhanced control of stray light.
Electromechanical optical modulators (sometimes called MARS modulators) are useful in optical communication systems. A electromechanical optical modulator is basically a Fabry-Perot cavity comprising the air gap between an optical membrane and a substrate. Modulation of reflected light is based on voltage-controlled movement of the membrane in relation to the substrate. Such devices can provide high contrast reflection modulation at rates in excess of several Mbit/sec. They are particularly useful as optical equalizers, switches for wavelength Add/Drop modules and optical cross-connect mirrors. U.S. Pat. No. 5,500,761 issued to K. W. Goosen et al. on Mar. 19, 1996 describes a electromechanical optical modulator useful for power equalization, and modulator having low insertion loss and enhanced operating bandwidth is described in the copending U.S. patent application Ser. No. 08/901,050 filed by K. W. Goosen et al on Jul. 25, 1997 and entitled xe2x80x9cMicroelectromechanical Modulator Having Enhanced Performancexe2x80x9d, now U.S. Pat. No. 5,870,221. Both U.S. Pat. No. 5,500,761 and application Ser. No. 08/901,050, now U.S. Pat. No. 5,870,221 are incorporated herein by reference.
Referring to the drawings, FIG. 1 is a schematic cross section of a conventional electromechanical modulator 9 comprising a substrate 10 and a membrane 15 spaced from the substrate to define an air gap 20. The substrate 10 is a conductive material such as doped silicon, and the has a top surface 21 (typically planar) and a back surface 22 (typically parallel or slightly inclined with respect to surface 21). The membrane 15 comprises one or more layers of conductive material such as an overlayer 15a of silicon nitride and an underlayer 15b of polycrystalline silicon. The overlayer has an index of refraction approximately equal to the square root of the substrate refractive index and the underlayer has an index of refraction approximately equal to the substrate refractive index. The thicknesses of layers 15a and 15b are each less than one-quarter of the operating wavelength xcex. The membrane 15 and the substrate 10 are spaced apart by a peripheral support layer 12 of insulating material. Electrodes 1 and 2 permit connection of the membrane 15 and substrate 10, respectively, to the terminals of a bias voltage source 29.
The air gap 20 can be controlled by a bias voltage between the substrate 10 and the membrane 15. Relative reflective maxima are produced when the gap 20 is an odd integer multiple of one-quarter of the operating wavelength xcex. Minima are produced when the gap 20 is 0 or an even integer multiple of xcex/4.
The modulator can employ mirrors of unequal reflectivity to provide broad operating bandwidth with low insertion loss. A high reflectivity membrane provides low insertion loss while a lower reflectivity substrate maintains the broader bandwidth of a low finesse device.
While these devices work well, stray light is a potential difficulty for some applications. Nonreflected light can be transmitted through the gap 20 into the underlying substrate. This light can be useful as a source of signals or information for feedback control, or it can be deleterious as a source of crosstalk through unwanted reflections or scattering. In either case there is a need to control the path of light transmitted into the substrate.
In accordance with the invention, a electromechanical optical modulator comprising an optical membrane, a substrate and Fabry-Perot air gap between them is provided with an improved structure for controlling light transmitted into the substrate. Specifically, an etched and coated cavity is formed in the backwall of the substrate underlying the air gap to receive transmitted light and redirect it onto controllable paths within the substrate. Advantageously the substrate is silicon, and the cavity is produced by anisotropic etching.